Horns are used as radiating elements in array antennas for fixed satellite service payloads. Typical fixed satellite service array antennas operate over fixed coverage regions using dual-linear polarizations. These array antennas are typically required to meet cross-polar isolation requirements of at least 30 dB over a relatively narrow bandwidth of 500 MHZ. However, there exists a need for array antennas having greater flexibility in terms of changing beam locations and/or reconfiguring beam shapes on orbit over a relatively higher bandwidth of 2000 MHZ to provide global reconfigurability.
A direct radiating array having a reconfigurable beam forming network is an ideal candidate for reconfigurable array antennas. In order to provide global reconfigurability, the array antenna has to scan roughly +/-9.degree. without the appearance of grating lobes in the visible angular region from the geostationary orbit of the satellite. A hexagonal grid arrangement of the radiating elements is preferred due to the reduction in the number of elements (about 15%) when compared with a square grid layout. A radiating element size on the order of three wavelengths is a desirable choice for minimizing the number of elements in the array antenna and pushing the grating lobes outside the +/-9.degree. field of view.
Using dual-mode circular horns as the radiating elements is undesirable because of limited bandwidth. Corrugated horns provide the necessary bandwidth but are not efficient when placed in an array because of wall thickness. Square horns provide the necessary bandwidth and meet the cross-polar requirements but are not suitable for the hexagonal grid arrangement.
Thus, there is a need for a rectangular horn suitable for use in an array antenna for dual-linearly polarized applications. Further, because the array antenna efficiency is improved by using multi-mode horns instead of dominant horns, there also exists a need for the rectangular horn to provide multi-modes.
Typical multi-mode rectangular/square horns use a step junction in one plane for supporting a single linear polarization, for instance, vertical polarization. The performance of these rectangular/square horns for the horizontal polarization is poor because the step junction is in the horizontal plane. In general, the multi-mode horns reported in the literature are efficient for an H-plane step junction but cannot be used for dual-linearly polarized applications.